The present invention relates in general to methods for determining the particle size and concentration in a fluid medium, and more particularly, it relates to a method for monitoring water filter performance and efficiency.
One of the major problems facing the water treatment industry today, is to determine the size distribution and concentration of particles in a water sample. A recent scientific review of the literature, relating to the evaluation of the performance of treatment plants, by means of particle counting, is published in "Evaluation of Particle Counting as a Measure of Treatment Plant Performance", by E. A. Hargesheimer et al., published by the AWWA Research Foundation in 1992.
To briefly summarize the conventional approaches listed in this publication, there are basically two general conventional ways to address the foregoing problem. The first and older approach, is referred to as the sedimentation rate study. This method includes sampling the water, and letting it settle for a certain period of time, for example one hour. Since larger particles tend to settle faster than smaller particles, the first sedimentation is made exclusively of larger particles. These particles are counted, and the water sample is allowed to settle for another period of time.
A second sedimentation, which includes smaller particles is similarly analyzed for particle concentration, and the above process of settling and counting is repeated until the desired particles of desired size are reached and counted. The use of the sedimentation rate study is no longer widely used, and is almost exclusively limited to specific applications. This approach is tedious, time consuming, and requires highly trained technical staff to count and to measure the size of each range of particles using a microscope.
The second approach is to electronically count the particles of particular sizes, within a sample, using such advanced technology as laser beams. However, this approach requires expensive counting equipment, highly trained personnel to operate and maintain the equipment. Additionally, this test takes a long time to implement in a laboratory environment.
While the conventional approaches have met with varying degrees of success in evaluating the water quality, the cost and complexity of the tests do not permit their widespread accessibility, particularly to smaller water treatment plants. This limited accessibility presents serious hinderance to the compliance with the federal and state laws, rules and regulations pertaining to water treatment standards.
On page 67 of the foregoing publication, the authors concluded that it "is not logical to seek a conversion factor to relate results from particle counting and turbidity since the level of discernment attainable by both methods is not comparable."
New laws are being continuously legislated to set water treatment and environment standards, and are becoming increasingly more stringent, due to the rising incidents of serious illnesses and deaths resulting from toxic suspended particles in water. Of special current concern are viruses and amoebic cysts, such as those of giardia and cryptosporidium.
The size of these viruses and cysts is in the order of 4 microns. Removal, and specially verification of such removal of particles of this size is expensive and has, so far, required special apparatus and trained personnel. Since the detection of viruses and cysts in very low concentrations is difficult and expensive, the rules "presume" that certain long standing technologies reduce the biological contaminants as required.
While large water treatment plants are presumed to comply, smaller treatment plants do not have the ready capability to evaluate the contamination problem, and consequently to comply with the new laws. One technology that could be used by small treatment plants is "slow sand filters". However, this alternative is still quite expensive, and requires highly trained, qualified and certified operators to conduct the tests.
Another type of filtration system that is commonly available at homes, is the "cartridge/bag filter", which is relatively inexpensive to operate and maintain. However, this type of filtration system presents its own concerns. To name a few, even if the cartridge bag filter is efficient in removing the undesirable particles, such as viruses and cysts, a leak somewhere in the water delivery system could render the filtration process futile.
One way to test the water delivery system, as a whole, would be to have a microbiologist determine the filter efficiency, by "spiking" the supply with the organisms to be removed.
Another way would be to have a specialist "challenge" the filter with four-micron size particles which serve as "surrogate" for the materials to be removed then to count the particles in the effluent to verify the required removal efficiency.
To that end, it would be highly desirable to have a new method for determining the filter efficiency. This method should be simple, inexpensive, readily accessible to permit compliance with the laws, and which would not require highly trained technical operators.